However, if you need to assign a different driver to the AM versus PM trip for example, you can still do this.     

Click on the Routes tab and edit the route that needs adjusting.

In this example, we will open the route for Bus 01 and click on the BUS 01 AM trip.  Once the trip is opened, click on the 3 dots to the right to edit the trip properties.

The Trip Properties pane will open at the bottom.  Click on the Driver tab in this pane.  The default driver will be based on the vehicle assigned to this route and the driver normally assigned to that vehicle.  To change the driver for this trip,  click in the Driver field and start typing.  A list of drivers will appear for you to select from.  

Select the Driver and click on the green Save icon.